Some sex workers who thought they were safe are now languishing in detention centres.

Until recently, Holbeck in Leeds was unlikely to see a news reporter from one end of the year to another. Lying on the southern edge of the city, Holbeck is partly residential, partly industrial, with a scattering of businesses: a tile shop, piano shop, printers, a tanning salon. Sex workers have hustled on these streets for years, but at the end of 2014 Holbeck was thrust into the limelight by a trial scheme: the UK's first "managed prostitution zone".

The managed area meant that sex work in Holbeck was now contained mainly in the business area, away from people's homes. Elsewhere in the UK, while selling sex is legal, soliciting is not. In the managed area, however, this law no longer applies between 7PM and 7AM. The basic premise is that, as long as you're over 18 – police may ask to see ID – you can work as a sex worker without fear of arrest.

On Friday the 21st of October, however, the truce appears to have broken. Women working in the area were shocked as police and UK Border Agency (UKBA) officials poured out of vans and cars and began to round up whoever they could find.

Anyone from outside the UK – and sex workers in the managed zone are frequently migrants – found themselves detained and questioned. After being interviewed on the street, six women, all from EU countries, were taken straight to Yarl's Wood and are now waiting to be deported. Others have been given 30 days to provide paperwork proving their legitimacy to stay in the UK.

Tanya, 23, is from Romania. She arrived at Yarl's Wood in the clothes she'd been working in and is now dressed in a regulation tracksuit.

"The night of the operation was awful," she says. "I didn't know what was happening. I had a good relationship with the police before. They asked if I was OK. It was normal; I'd joke with them. I knew the police were around, and I'd call them if I needed them."

Tanya says that, the same night she was detained by the UKBA, she'd called the police after a client refused to take her back to the managed area. "I was scared," she says. "So I rang the police and they spoke to the punter and stayed on the phone to me while the punter finally agreed to drop me back."

Like the other women detained that night, Tanya says she isn't sure why she's being deported. "I'm not a criminal," she says. "I don't take any benefits. I keep myself to myself. It's like prison here. No one tells you anything. You just wait and wait and wait. I don't know when I'll go home. I could be here one month, two months, six months."

The Home Office declined to comment on the cases, but it's likely the women are being deported for failing to observe their EU treaty rights. EEA nationals who've been in the UK for longer than three months need evidence either that they're working, studying or that they're financially self-sufficient. Without payslips, a contract, proof of National Insurance contributions or bank statements, for example, you can be kicked out.

Sex workers are easy targets for immigration stings. It's cash-in-hand work that requires no papers and can be done with little English. Despite being legal, it's regarded as shady and illegitimate. Who's going to complain if a few hookers get sent home? No coincidence, perhaps, that the same week women in Leeds were detained, similar raids took place in London and Bolton.

Basis Yorkshire works with Safer Leeds (a West Yorkshire Police and Leeds City Council partnership) in the managed area. The charity is horrified by what took place on the 21st of October.

"Since working under a managed approach, the relationship with the police has been substantially improved," Basis said in a statement. "Amongst other things, this has led to the identification and prosecution of criminals, including those guilty of sexual and other violent forms of assault on sex workers and other citizens in Leeds. We believe strongly that [the UKBA / police operation] has been detrimental to the relationship between street sex working women and the police."

No one is pretending the managed zone is an idyll. Last year, Daria Pionko, a Polish woman, was murdered while working here. Her killer is now serving life in jail. Other attacks have taken place. But what's different from other areas is that women have been willing to speak to the police and prosecutions have been made. Some local residents and business owners say they feel unsafe and that, although having sex in public and littering is illegal in the managed area, it still takes place.

Media reports of the managed area are frequently lurid and offer conflicting views, but a report by researchers at the University of Leeds suggests that, since the scheme was introduced, crimes are more likely to be reported and prosecutions made, and that locals business owners and residents were largely supportive of the scheme.

For the police, this is a balancing act. Lots of the women working in Holbeck are already vulnerable and would remain so were the managed zone to disappear. Their safety must be weighed up alongside the feelings of non-sex working locals.

A spokesperson for Safer Leeds told me: "Attention is now focused on providing increased support to vulnerable sex workers with complex needs, particularly addictions, to exit this work. Adjustments are also being made to the rules and operation of the Managed Approach, which sit aside targeted enforcement tactics with partner agencies."

It's not known what these adjustments will be, but the targeting of migrant women seems incompatible with an approach that prioritises sex workers' safety.

Monika, from Hungary, is also in Yarl's Wood. She's confused at the conflicting messages; the fact that, just two weeks ago, police in Holbeck were chatting to her and asking if she was OK. Now she's here.

"I was told I wasn't being taken to a prison, but it feels like I'm in prison," she says. "The doors are locked. I never realised I wasn't allowed to be working [in Holbeck]. Now I don't know what's going on. I feel like I'm going crazy."